Epilepsy
Epilepsy is the fourth most common neurological condition, after migraines, strokes, and Alzheimer's Disease. It is a very serious condition, resulting in considerable morbidity and mortality.
Epilepsy affects an estimated 1% of the population, which equates to about 60 million individuals worldwide. In the UK alone, over half a million people have epilepsy, around 1 in 100 people.
Anyone can develop epilepsy, at any time of life. Patients usually experience debilitating seizures associated with abnormal electrical activity in the brain. Seizures are a combination of electrical and behavioural events that can induce chemical, molecular, and anatomic alterations. In some cases, the seizures can be accompanied with transient behavioural changes and a range of comorbidities including cognitive impairments, anxiety and systemic effects.
At least 15 different types of seizures and 30 epilepsy syndromes have been identified.
There remain many unresolved clinical issues around epilepsy. Pharmacological therapy remains unsatisfactory for many. None of the anti-epileptic drugs currently used can completely prevent the development of epilepsy and the progression of the disease. They can also cause quite debilitating side effects. Moreover, about one third of the patients treated with antiepileptic drugs continue to experience seizures. This is without counting that there is a lack of therapies that can ameliorate or prevent the associated cognitive, neurological and psychiatric comorbidities, or the epilepsy-related mortality.
Consequently research into epilepsy is still very much needed to improve the quality of life of patients and to prevent the disease from taking its toll of patients. Seizure prediction, treatment for drug-resistant epilepsy, and the perspective of epilepsy genetics still remain major challenges.
Animal models of epilepsy
Understanding of the complex mechanisms underlying epileptogenesis and seizure generation cannot be fully acquired in clinical studies with humans. Epilepsy research has a long history of comparative anatomical and physiological studies on a range of mostly mammalian species. Research into the origins and mechanisms of epilepsy advanced for the most part quite quickly thanks to this animal research.
Seizures can be induced in animals to investigate both the mechanism of epilepsy and help identify new treatments.
Animal models of epilepsy and seizures are used to understand the pathophysiology of the different forms of epilepsy and their comorbidities, to identify biomarkers, and to discover new antiepileptic drugs to prevent the epileptogenic process, better treat comorbidities and treat drug resistant epilepsy.
Mice and rats are the most common animal models of epilepsy, but there are a large diversity of acute and chronic mammalian models that have been developed across time. More complex mammalian brains and genetic model organisms including zebrafish have been studied less, but offer substantial advantages that are becoming widely recognized.
It is important is bear in mind that there is a translational gap between animal data and clinical trials, not least because the human brain is immensely complex. However, animal research has already increased the basic knowledge of the disease which has helped our understanding of the biological aetiology of seizures, genetic correlates, and related comorbidities of epilepsy, and well as having applications for epilepsy treatment.
Inducing epilepsy in animals
Procedures leading to the induction of seizures and/or epilepsy should be tailored to reach the scientific endpoint effectively whilst limiting suffering and mortality.
Epilepsy can be induced several ways in the animal. These may involve neurochemical agents, electrical stimulation, thermal or hypoxic insults, traumatic injuries, optogenetics, and rodent strains with idiopathic or audiogenic-induced seizures. Such a number of experimental options probably reflect the diversity of seizure types in humans.
Chemoconvulsants allow for rapid investigation of epileptogenic mechanisms and drug screening at the expense of high mortality and high variability in the frequency and severity of spontaneous seizures. However, these models are not completely clinically validated, as they are not predictive of clinical response to all therapeutic strategies. The choice of a given model should, therefore, rely on which group of specific features one is aiming to study.
- Electrical stimulation protocols are less harmful and offer better control of seizures, as they reproduce epileptogenic features in the intact brain with low mortality and high reproducibility. Unlike chemical-induced seizures, postictal alterations* from electrical stimulation can be investigated when the epileptogenic cause is no longer present. However, these models do not provide cell-type specificity in the brain and can be costly laborious and time-consuming especially when used for chronic studies.
- Non-chemical and non-electrical insults such as reducing oxygen supply might approximate clinical conditions of developmental epilepsies, but inconsistencies in seizure susceptibility depending on experimental procedures hamper comparisons between rodents and humans.
- Seizure-prone, often genetically modified, strains eliminate much of the artificiality of experimentally induced seizures, but their genetic alterations are not fully known. Sensorial triggers are often also needed to induce seizures in some of the strains.
Models can be distinguished between acute and chronic, focal and generalised, acquired and genetic.
- Acute animal models model symptomatic seizures in normal brain. Acute animal models use chemical, electrical or physical stimulation to trigger epileptic activity and have been extensively used to discover new drugs but they have not led to the development of drugs that target either the generation or the maintenance of the epileptic state.
- Chronic epilepsy results from pathological structural and functional changes in the brain. Chronic models have been recommended for many kinds of epilepsy research including screening systems.
Unfortunately, some of the procedures used to induce, maintain and monitor seizures can be distressing for the animals. They have the potential to cause pain, suffering, distress and lasting harm to the animals involved. Behavioural comorbidities such as cognitive impairments, hyperactivity or depression are an integral part of the epilepsy syndrome. Modelling the disease often causes the animals to experience them. This can and does impact the animals’ welfare. Consequently researchers and ethical oversight committees must carefully weigh the potential research benefits against the harms to the animals.
In the UK and Europe such models are typically classified in legislation (e.g. the European Directive on the protection of animals used for scientific purposes (2010/63/EU)) as moderate or severe procedures. There is therefore a need for clear guidance on their use and refinement, in order to minimise any suffering, which is important for ethical, scientific, and legal reasons.
Choosing the right animal model
The choice of model depends on the type of epilepsy being modelled, the scientific question being asked, and the need to minimise animal suffering and numbers.
Models should represent key features of the corresponding disease. However, each model shouldn’t necessarily strive to be identical. For example, researchers may need a higher rate of seizures than occurs naturally to improve the power of the experiment. Testing low epileptic rates may be impractical in scientific settings and timelines.
Variations in the strain, genetic background, source, age and sex of animals can influence seizure susceptibility and mortality.
Choice of strain
Rodents are the models of choice for the study of cellular and neural network mechanisms underlying epilepsy. However, mouse genetic backgrounds play a crucial role in phenotypes and the susceptibility of these strains to seizures and neuropathological consequences. Different rodent strains may differ in seizure susceptibility, effects of antiepileptic drugs, as well as the consequences of seizure activity. It is, thus, important to choose carefully the right strains for each experiment and even identify individuals with same characteristic within a colony.
Choice of age
Seizure susceptibility and manifestation can be age-dependent. Some types of seizures and epilepsies occur in neonates and infants, others only in adults. Moreover, brain functions such as neurotransmission, neuronal properties and connectivity change during development. As such, the age of the animals is likely to affect factors such as sensitivity to chemoconvulsants, seizure latency and intensity, mortality and behavioural, pathophysiological and pharmacological responses to anticonvulsants.
Choice of sex
Sex hormones may influence the timing and frequency of certain seizures. Gender differences are emerging amongst some types of epilepsies (Galanopoulou, 2014). Females may be more susceptible to certain types of epilepsies whilst males are more susceptible to others.
Recommendations
Variations in seizure susceptibility and mortality should be taken into consideration when designing and conducting studies to reduce experimental bias. The strain, source, age and sex of animals used in studies should be consistent, and reported in publications. Moreover, given the evidence for sex-specific effects on epileptogenesis, consideration should be given to using animals of both sexes.
Welfare assessment and implementing the 3Rs
Animal models play a key role in epilepsy research but their use is associated with considerable welfare cost to the animals involved. It is important that researchers, by any means necessary, minimise pain, suffering and distress of the animals to improve animal welfare and optimise the quality of studies in the field.
The creation of chronic disease conditions, including rodent models of epilepsy, requires continuous welfare assessment. In addition to providing an immediate assessment of the animal's state of health and welfare, the information gained from welfare assessments enables compliance with the legal requirements in many countries for prospective severity classification of the experimental procedures and subsequent retrospective assessment of severity experienced by the animals (see Annex VIII of Directive 2010/63/EU).
One of the major welfare concerns for those working with animal models of epilepsy is the experience of the animal during and between seizures. It can be difficult to appreciate the experience of animals during seizures as it can be witnessed very differently from what happens in patients. Choosing a model solely based on the frequency or intensity of seizure activity the animals experience may not be appropriate. The priority should beto objectively assess the experience of the animal and think critically about which model is most relevant in regards to the scientific question.
Assessment of animal welfare, and balancing harms to animals against the potential benefits of the research, must address the whole epilepsy syndrome, not just one of the clinical signs.
For more information, consult the NC3Rs website.
Recommendations to improve animal welfare in rodent models of epilepsy and seizures can also be found here : https://www.sciencedirect.com/science/article/pii/S0165027015003404#bib0965
Non animal models
Animal models of epilepsy have dominated the landscape with regard to research and development. Advancements in the understanding of epilepsy physiopathology and treatment has largely depended on the use of animal models.
However, these animal models cause welfare concerns and there is an unavoidable translational gap to consider so researchers have tried to develop alternatives to the use of animal models of epilepsy.
A compelling alternative is the use of live human epileptic tissue. A select number of laboratories worldwide are increasingly using brain slices of living human tissue resected during surgery to perform functional mechanistic studies. The use of live epileptic human tissue offers unprecedented opportunities to understand, in particular, the mechanisms associated with difficult to treat epilepsy whilst also permitting studies of efficacy of novel agents that are being developed to alleviate epilepsy in drug resistant patients.
Another alternative is the use of Dictyostelium (slime-moulds). This non-sentient biomedical model has been used as a replacement for animals in understanding the basis of epilepsy and the cellular effects of epileptic drugs. It has notably helped develop new epilepsy treatments and refine animal usage to prove the efficacy of these drugs.
Today, testing for a new epilepsy drug employs at least two experimental animal models at five drug doses with eight animals per dose. This amounts to around 100 animals per compound. Including Dictyostelium in the process can reduce the need of animals to 25 individuals to screen for acompound. So if you were screening forty drugs, using Dictyostelium would reduces animal usage in the development of these drugs by 3900.
These are only a few examples. Other such methods are being developed to reduce the number of animals used in epilepsy research. For more information visit the NC3Rs website and their dedicated page.
* The postictal state is the altered state of consciousness after an epileptic seizure. It usually lasts between 5 and 30 minutes, but sometimes longer in the case of larger or more severe seizures, and is characterized by drowsiness, confusion, nausea, hypertension, headache or migraine, and other disorienting symptoms.
Last edited: 15 February 2023 09:21